JP2015511335A - Optical mode conversion using inter-mode Cherenkov radiation - Google Patents

Abstract

Embodiments of the present invention generally relate to optical mode conversion using inter-mode Cherenkov radiation. More particularly, embodiments of the present invention utilize inter-mode four-wave mixing to convert light between modes for composite applications, whereby one of the four waves is generated by Cherenkov radiation. It relates to the generated optical mode conversion. According to one embodiment of the present invention, the fiber outputs an input end for receiving light in a first mode at a first wavelength and a desired second mode at a desired second wavelength. And the desired second mode is controlled by deformation of the fiber, such as by bending, during the inter-mode Cherenkov radiation process.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority to US Provisional Patent Application No. 61 / 601,110, filed February 21, 2012, entitled “Intermodal Cherenkov Radiation”, the entire disclosure of which is incorporated herein by reference. Which is incorporated herein by reference. This application is also a PCT international patent claiming priority from US patent application Ser. No. 13 / 945,475, filed Oct. 9, 2012, filed Dec. 6, 2012 at the US receiving Office. This is a continuation-in-part of application PCT / US12 / 68259, the entire disclosure of which is incorporated herein by reference.

  Embodiments of the present invention generally relate to optical mode conversion using inter-mode Cherenkov radiation. More specifically, embodiments of the present invention utilize inter-mode four-wave mixing to convert light between modes for composite applications, whereby one of the four waves is generated by Cherenkov radiation. It relates to the generated optical mode conversion.

  Fibers that support higher order mode (HOM) light propagation have recently received significant attention due to the possibility of increasing data transmission capacity through mode division multiplexing. Nonlinear effects of HOM fibers such as dispersion wave generation have also been studied as an effective technique for nonlinear wavelength conversion. However, previous designs of HOM fibers have always tried to avoid mode crossing at the operating wavelength (ie, two propagation modes have the same propagation constant at the same wavelength).

  With the use of optical HOM fibers, it is desirable for many applications to have some way of coupling / converting light from one mode to another. There are many known ways to achieve such conversion of light based on a linear conversion process. Known linear conversion methods include the use of conventional mode converters such as long-period gratings with fibers, phase plates, or spatial light modulators.

In a typical linear mode conversion process, light can be supplied in the fundamental mode (LP ₀₁ ). The light is then passed through a conventional mode converter and converted to a different mode, for example LP ₀₂ . When using a conventional linear mode converter, the converted light is generated at the same wavelength as the input light, which is acceptable or desirable for many applications. However, in some cases, the converted bandwidth can be generated at a different wavelength than the original input light, thereby increasing the operating bandwidth for a given application.

  Four-wave mixing is an intermodulation phenomenon of nonlinear optics. Usually, four-wave mixing is utilized by various wavelengths within the same mode. Thus, to achieve mode conversion using a four-wave mixing process, a diffraction grating or other form of known mode converter is conventionally required.

  Therefore, in order to pioneer a vast new application field that can be further expanded using signal processing such as wavelength conversion, parametric amplification, high-speed switching, optical sampling, pulse compression, optical regeneration, etc., nonlinear mode conversion A method is needed.

  Embodiments of the present invention generally relate to optical mode conversion using inter-mode Cherenkov radiation. More specifically, embodiments of the present invention utilize inter-mode four-wave mixing to convert light between modes for composite applications, whereby one of the four waves is generated by Cherenkov radiation. It relates to the generated optical mode conversion.

  According to one embodiment of the present invention, the fiber has an input end for receiving light in a first mode at a first wavelength and light in a desired second mode at a desired second wavelength. And the desired second mode is controlled by bending the fiber during the inter-mode Cherenkov radiation process.

  In another embodiment of the invention, a system for non-linear mode conversion is a light source for providing input light in a first mode at a first wavelength and a fiber, and a first at a first wavelength. An input end for receiving light in a mode and an output end for outputting light in a desired second mode at a desired second wavelength, wherein the desired second mode is inter-mode Cherenkov radiation Fiber, controlled by bending the fiber during the process.

  In yet another embodiment, a method of nonlinear mode conversion of light provides a light source capable of producing input light in a first mode at a first wavelength and for receiving input light from the light source. Providing a fiber having an input end and an output end for outputting output light in a desired second mode at a desired second wavelength; generating input light with a light source; and mode Utilizing the means for the inter-Cerenkov radiation process, the desired second mode being controlled by the inter-mode Cherenkov radiation process.

  In order that the above-listed features of the invention may be understood in detail, a more detailed description of the embodiments of the invention briefly summarized above may be had by reference to the embodiments illustrated in the accompanying drawings. . However, the accompanying drawings show only typical embodiments of the embodiments included within the scope of the present invention, and therefore the present invention can recognize other equally effective embodiments. Note that this should not be considered limiting.

4 is a plot of effective refractive index as a function of straight fiber versus bent fiber wavelength, in accordance with an embodiment of the present invention. FIG. 6 is a plot of phase matching conditions between LP ₁₁ mode Cherenkov radiation at ω _c and LP ₀₂ mode pump solitons at ω _p according to one embodiment of the invention. Curves of propagation constants of LP ₀₂ and LP ₁₁ modes shown near the mode crossing frequency ω _x using the effective refractive indices of the LP ₀₂ and LP ₁₁ modes of the HOM fiber according to one embodiment of the present invention (left ) And a dispersion curve (right) of the HOM fiber. 1 is a schematic diagram of a system for generating inter-mode Cherenkov radiation, according to one embodiment of the invention. FIG. Soliton at different input pulse energies in a straight fiber (left), a 14 cm radius wound fiber (middle), a 5.5 cm radius wound fiber (right), according to an experimental embodiment of the present invention A graph showing the measured spectrum of shift and Cherenkov radiation. FIG. 6 is a graph showing the corresponding optical spectrum and spatial profile (left) of soliton and Cherenkov radiation and the measured intensity autocorrelation trace (right) of Cherenkov radiation, according to an experimental embodiment of the present invention.

  The headings used herein are for organizational purposes only and are not meant to be used to limit the description or the claims. As used throughout this application, the word “may” is not in a compulsory sense (ie meaning that it must be done) but in an acceptable sense (ie meaning that it has potential). used. Similarly, the terms “include”, “including”, and “includes” mean including but not limited to. For ease of understanding, like reference numerals have been used, where possible, to designate like elements that are common to the figures.

  Embodiments of the present invention generally relate to optical mode conversion using inter-mode Cherenkov radiation. More particularly, embodiments of the present invention utilize inter-mode four-wave mixing to convert light between modes for composite applications, whereby one of the four waves is derived from Cherenkov radiation. It relates to the generated optical mode conversion.

  As used herein, the term “about” or “approximate”, or derivatives thereof, when referring to a numerical value, should be considered to include within 10 percent of such numerical value in both directions. It is. In addition, when such terms are used to describe an absolute value (eg, zero), the absolute value is within one unit of a reasonable measurement in both directions, as is commonly used by those skilled in the art. Should be considered as including.

  Cherenkov radiation in nonlinear optics is also known as non-soliton radiation, a special form of four-wave mixing where the soliton is phase matched with the dispersive wave. In single mode operation, the dispersed wave is generated in the normal dispersion region. The frequency shift between solitons and dispersive waves in optical fibers is similar to the angle at which Cherenkov radiation is emitted by charged particles in a bulk medium.

  According to embodiments of the present invention, designing a higher order mode (HOM) fiber having at least a first and a second mode related to a mode conversion process, and an initial input mode (ie, a first mode) And the wavelength of the desired converted output mode (ie, the second mode) may not be the same. Embodiments of the present invention can be used to generate light of different modes and wavelengths without being limited to the wavelengths of the initial input mode.

  In addition, in some embodiments, including four different modes in a single four-wave mixing process, and multiple four-wave mixing processes are applied, whereby at least one of each of the four-wave mixing processes is applied. If the wave is generated from Cherenkov radiation, it may be feasible to include multiples thereof. In the Cerenkov radiation process, there are only two separate waves of interest, namely solitons and dispersive waves, which are the same mode in the commonly known use of Cherenkov radiation. However, the two components, soliton and dispersive wave, exist in two different modes and can result in inter-mode Cherenkov radiation. In other embodiments, it may be possible to have a series of different inter-mode Cherenkov processes by further performing the processes described herein.

If two optical modes of an optical fiber at a wavelength have the same effective refractive index, this is called a mode crossing. According to one embodiment of the invention, a fiber with mode crossing can be designed by providing the fiber with a core and a ring separated from the core by a trench. Such a design is sometimes known as a triple-clad design. Depending on the design, for a given wavelength range, the LP ₀₂ mode of such a fiber may be more ring-bound at short wavelengths and more core-bound at longer wavelengths. _Eleven modes are bound to the ring over the entire wavelength range considered. Thus, in such a fiber, the LP ₀₂ mode will have a larger effective refractive index than the LP ₁₁ mode at shorter wavelengths. Therefore, since the LP ₀₂ mode shifts from the ring mode to the core mode at a longer wavelength, there may be a mode crossing between the LP ₀₂ mode and the LP ₁₁ mode. When LP ₁₁ is in ring mode and LP ₀₂ is in core mode, LP ₁₁ mode is better bound to the core than LP ₀₂ mode, and therefore LP ₁₁ mode has a higher effective refractive index, so Such mode crossing occurs.

  In non-linear four-wave mixing schemes, four waves are used to create non-linear coupling, and are generally phase matched with non-zero transversal field overlap to achieve strong coupling. The Waves used in the four-wave mixing process can include either continuous waves or pulse waves. In one embodiment of the present invention, such a mixing process utilizes a third order nonlinear interaction between the doped silica material and the light wave.

  All four waves of a typical four-wave mixing process can have different wavelengths, but such different wavelengths are not required. In one embodiment, each of the waves is in the same mode and thus will have the same angular symmetry and can be easily mixed. In another embodiment, one of the waves is a mode that has a different angular symmetry than the three other waves, and thus there is no coupling between all of the waves. In yet another embodiment, there are two waves having a mode with the same angular symmetry between two waves and two other waves having a mode with different angular symmetry. In such embodiments, coupling between each of the waves is possible if the phase matching requirements are met.

  Within an optical non-linear community, Cherenkov radiation is derived from three soliton waves and four waves from Cherenkov radiation, or three waves from Cherenkov radiation, It is well known that in the type of four-wave mixing where the fourth wave is a soliton, it can be generated from the soliton. Cherenkov radiation is a dispersive wave that is phase matched to the wave from the soliton. In general, soliton and Cherenkov radiation are the same light mode. However, in embodiments of the present invention, the soliton pulses that produce Cherenkov radiation need not be in the same mode as long as they can meet the phase matching requirements in addition to non-zero lateral field overlap.

  According to embodiments of the present invention, by designing an optical fiber to have a mode crossing at a particular wavelength of interest, by deformation of the optical fiber, for example, by bending the fiber, the inter-mode Cherenkov radiation process Through what is known, it is possible to achieve both wavelength and mode conversion.

FIG. 1 represents a plot of effective refractive index as a function of straight fiber versus bent fiber wavelength according to an embodiment of the present invention. The bent fiber introduces a tilted index profile for each mode. It can be noticed that there is no mode crossing in the bent fiber. Thus, the bent fiber soliton is coupled to the dispersive wave via a Cherenkov radiation process when both the soliton and the dispersive wave are in the same mode of the bent fiber. When the fiber is straightened, the soliton light in the bent fiber mode will be in the LP ₀₂ mode and the dispersive wave will be in the LP ₁₁ mode. Thus, in a bent fiber, the intermode coupling is simply a normal single mode Cherenkov coupling.

  As a result, by bending the fiber, the angular symmetry of the mode is broken and there is no longer any orthogonality of angular symmetry between the waves. Thus, when bent, it is possible to create a coupling between the two modes, in which the straightened fibers have different angular symmetry and are therefore orthogonal.

  Any known technique can be used to bend the fiber or similarly deform the profile to achieve substantially the same result. For example, other methods use a heat source to use a microbend grating, create a temperature gradient across the refractive index profile, and produce the same tilde refractive index profile as bent. Using a single stress rod in the fiber to create an asymmetric stress profile, using a fiber with an inserted electrode to create an electrically polarized fiber, or a waveguide When made of a different material than the fiber, the tilde index profile can also be created in the silicon waveguide, for example, by the electric field generated by it.

  It is further necessary to have phase matching between the two modes for efficient coupling. Such phase matching can occur in straight fibers indicating that the two modes have mode crossings and the propagation constants are almost the same for the two modes. In addition, it suggests that the propagation constant is relatively decreased for one mode and the propagation constant is relatively increased for the other mode. The phase matching equation can be written as:

Here, β is a propagation constant, and ω is a frequency. The subscripts indicate the pump soliton (p) and Cherenkov radiation (c) in LP ₀₂ mode (02) and LP ₁₁ mode (11), respectively. v _g = (dβ / dω) ⁻¹ is the group velocity of the soliton. β (ω) can be derived from the effective refractive index (n _eff ) of the fiber as a function of ω. P and γ are the soliton peak power and nonlinear coefficient, respectively. When the peak power is about 10 kW or less, the nonlinear contribution represented by the last term of equation (1) can generally be ignored. Based on equations (1) and (2),

Is

become.

The solution of Equation 3 is examined and shown using the graph in FIG. FIG. 2 represents a plot of phase matching conditions between LP ₁₁ mode Cherenkov radiation at ω _c and LP ₀₂ mode pump solitons at ω _p according to one embodiment of the present invention. If the soliton propagates with anomalous dispersion (ie, β _{p, 02} has negative curvature) at ω _p higher than the mode crossing frequency ω _x , the solution is found at ω _c lower than ω _x satisfying Equation 3. be able to. Accordingly, phase-matched Cherenkov radiation can be generated.

FIG. 3 illustrates the LP ₀₂ and LP ₁₁ mode propagation constants shown in the vicinity of the mode crossing frequency ω _x using the effective refractive indices of the LP ₀₂ and LP ₁₁ modes of the HOM fiber according to one embodiment of the present invention. The schematic diagram of the curve (left) and the dispersion curve (right) of the HOM fiber is shown.

  By changing the bend radius of the fiber, the greater the deformation of the fiber, the more the angular symmetry is broken, so the strength of the nonlinear coupling can be controlled. Using fiber bending, it may also be possible to tune the wavelength of the mode to be converted, so that the wavelength at which phase matching exists is also perturbed. In an alternative embodiment, the deformation of the optical fiber does not necessarily have to occur by bending the fiber, i.e. other means of deformation that break the angular symmetry are also supported.

Experimental Embodiment In one exemplary embodiment described in more detail below, an experimental setup such as the system shown in FIG. 4 is provided for inter-mode Cherenkov radiation of HOM fiber. As shown in FIG. 4, the experiment included means for recording the process, such as a light source, variable optical attenuator, HOM fiber, optical spectrum analyzer, and camera.

In the experiments performed, the light source was an IMRA FCPA μJewel ™ D400 laser that outputs a 1 MHz pulse train at 1045 nm in free space. In such a laser, the pulse has an autocorrelation FWHM of 600 fs. The power coupled to the fiber is controlled by a variable optical attenuator consisting of a half-wave plate and a linear polarizer. With the achromatic objective, the light was coupled to a 90 cm HOM fiber attached to a triaxial stage. The fiber end position is optimized to achieve maximum excitation in the LP ₀₂ mode. The output of the HOM fiber was characterized with an optical spectrum analyzer and a second order interference autocorrelator. Next, the spatial profile of the fiber output was magnified 250 times with a 4f imaging system and recorded with a CCD camera.

  When considering the intended operating parameters, the desired refractive index profile of the HOM fiber was constructed. Such a construction process is described in detail in US patent application Ser. No. 13 / 945,475, filed Oct. 9, 2012, owned by the rightful owner of this application, the entire disclosure of which is hereby incorporated by reference. Is incorporated herein.

  With respect to the refractive index profile of the fiber, the fiber includes a central core, an inner trench surrounding the core, a ring surrounding the trench, an outer trench surrounding the ring, and an outer cladding.

In some embodiments, the central core of the fiber generally has a radius between about 0.75 μm and about 2.0 μm and between about 20.0 and about 40.0 (measured at 10 ⁻³ ). Can have a raised refractive index region with a refractive index difference relative to the outer cladding. In one exemplary embodiment, the central core can include SiO ₂ doped with an appropriate amount of GeO ₂ to achieve a desired refractive index, but can include other dopants.

The inner trench has a width between about 1.75 μm and 2.5 μm and a refractive index difference with respect to the outer cladding between about −3.0 and about −13.0 (measured at 10 ⁻³ ). The refractive index region can be lowered. The inner trench can generally include SiO ₂ doped with an appropriate amount of F and optional GeO ₂ to achieve the desired refractive index.

The ring has a width between about 2.0 μm and about 5.0 μm and has a refractive index difference with respect to the outer cladding between about 5.0 and about 20.0 (measured at 10 ⁻³ ) It can be a refractive index region. The ring can generally include an appropriate amount of GeO ₂ and optionally F-doped SiO ₂ to achieve the desired refractive index.

The outer trench generally has a width of about 1.75 μm to about 4.5 μm and a refractive index difference relative to the outer cladding of between about 1.5 and about −3.5 (measured at 10 ⁻³ ). The outer trench can generally include SiO ₂ doped with an appropriate amount of P ₂ O ₅ , F, and optionally GeO ₂ to achieve the desired refractive index.

In many embodiments, the outer cladding comprises SiO ₂ and has an outer radius between about 50 μm and about 75 μm.

  One exemplary first designed fiber specific design is shown in Table 1 below.

The experimental HOM fiber was designed to produce LP ₀₂ mode high energy solitons. As designed, the soliton self-frequency shift causes the soliton to shift red toward the mode crossing wavelength and excite LP ₁₁ mode Cherenkov radiation. In addition, the designed experimental fiber sets LP ₀₂ and LP ₁₁ modes with anomalous and normal dispersion, respectively, between 1000 nm and 1200 nm, with a mode crossing wavelength of 1120 nm.

As a result of the experiment, high energy soliton 1085nm is, a propagation distance of the input pulse energy and 90cm of 14.4NJ, generated by the _{LP 02} mode of the fiber. In a straight HOM fiber, the soliton center wavelength could be tuned continuously between 1085 nm and 1120 nm. It was observed that mode coupling induced by fiber bending caused spectral modulation in 1120 nm solitons. However, LP ₁₁ mode Cherenkov radiation has not been observed with straight fibers. Such observations were consistent with the theoretical prediction that the mode overlap integral would disappear without significant perturbation.

  FIG. 5 is different for a straight fiber (left), a fiber wound with a 14 cm radius (center), and a fiber wound with a 5.5 cm radius (right), according to an experimental embodiment described herein. Fig. 4 represents the measured spectrum of soliton shift and Cherenkov radiation at the input pulse energy. This figure shows that by bending the HOM fiber, the mode overlap integral no longer disappears, thereby providing inter-mode nonlinearity for efficient generation of Cherenkov radiation. At 18 nJ input energy, Cherenkov radiation can be clearly observed at wavelengths longer than 1130 nm for bend radii less than 14 cm. It is also clear from FIG. 5 that the wavelength separation between Cherenkov radiation and solitons increases with decreasing bend radius. As a result, experimental results show that the spectrum of Cherenkov radiation can be effectively adjusted by controlling the deformation to the fiber. However, it should be noted that the difference in input polarization does not affect the spectral evolution and wavelength of Cherenkov radiation, indicating that the inter-mode Cherenkov radiation created by waveguide bending is not a polarization effect. .

As a result of the experiment, Cherenkov radiation was observed in a HOM fiber with a 5.5 cm bend radius. At 21 nJ input energy, a 1093 nm 6 nJ soliton excites a 1140 nm LP ₁₁ mode 1.5 nJ Cherenkov pulse. Such a measured wavelength of Cherenkov radiation is consistent with the solution of Equation 3 above, based on the pump soliton n _eff value and wavelength.

FIG. 6 is a graph showing the corresponding optical spectrum and spatial profile of soliton and Cherenkov radiation (left) and the measured intensity autocorrelation trace (right) of Cherenkov radiation, according to the experimental embodiment described herein. Represents. According to the intensity autocorrelation measurement, the pulse width of Cherenkov radiation is 5.8 ps. The Cherenkov pulse spreads due to the dispersion of the LP ₁₁ mode and the temporal walk-off between solitons and Cherenkov radiation. Thus, experiments show that simultaneous wavelength and mode conversion can be achieved through the use of inter-mode Cherenkov radiation.

  Although the foregoing relates to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. It is further understood that the various embodiments described herein can be utilized in combination with any other described embodiments without departing from the scope included herein. In addition, embodiments of the present invention may be further extensible when specific applications may be required.

Claims (10)

An input end for receiving light in a first mode at a first wavelength;
A fiber including an output end for outputting light in a desired second mode at a desired second wavelength,
A fiber, wherein the desired second mode is controlled by deformation of the fiber during an inter-mode Cherenkov radiation process.   The fiber of claim 1, wherein the fiber comprises a higher order mode fiber.   The apparatus further comprises means for applying inter-mode four-wave mixing to convert the first mode at the first wavelength to the second mode at the desired second wavelength. Fiber as described in 1.   The fiber according to claim 3, wherein at least one of the waves of the four-wave mixing process is a Cherenkov radiation wave.   The fiber of claim 3, wherein the phases of the modes associated with the inter-mode four-wave mixing process are matched.   The fiber of claim 3, wherein the four-wave mixing process maintains a non-zero lateral field overlap. A light source for supplying input light in a first mode at a first wavelength;
Fiber,
An input end for receiving light in a first mode at a first wavelength;
An output end for outputting light in a desired second mode at a desired second wavelength,
A system for nonlinear mode conversion comprising a fiber wherein the desired second mode is controlled by the bend radius of the fiber during an inter-mode Cherenkov radiation process.   The system of claim 7, wherein the fiber comprises a higher order mode fiber.   8. The method of claim 7, further comprising means for applying inter-mode four-wave mixing to convert the first mode at the first wavelength to the second mode at the desired second wavelength. The described system. Providing a light source capable of generating input light in a first mode at a first wavelength;
Providing a fiber having an input end for receiving the input light from the light source and an output end for outputting output light in a desired second mode at a desired second wavelength;
Generating the input light with the light source;
Using a means for the inter-mode Cherenkov radiation process, comprising:
The method wherein the desired second mode is controlled by the inter-mode Cherenkov radiation process.